In order to find and produce hydrocarbons e.g. petroleum oil or gas hydrocarbons such as paraffins, naphthenes, aromatics and asphaltics or gases such as methane, a well may be drilled in rock (or other) formations in the Earth.
After the well bore has been drilled in the earth formation, a well tubular may be introduced into the well. The well tubular covering the producing or injecting part of the earth formation is called the production liner. Tubulars used to ensure pressure and fluid integrity of the total well are called casing. Tubulars which bring the fluid in or from the earth formation are called tubing. The outside diameter of the liner is smaller than the inside diameter of the well bore covering the producing or injecting section of the well, providing thereby an annular space, or annulus, between the liner and the well bore, which consists of the earth formation. This annular space can be filled with cement preventing axial flow along the casing. However if fluids need to enter or leave the well, small holes will be made penetrating the wall of the casing and the cement in the annulus therewith allowing fluid and pressure communication between the earth formation and the well. The holes are called perforations. This design is known in the oil and natural gas industry as a cased hole completion.
An alternative way to allow fluid access from and to the earth formation can be made, a so called open hole completion. This means that the well does not have an annulus filled with cement but still has a liner installed in the earth formation. The latter design is used to prevent the collapse of the bore hole. Yet another design is when the earth formation is deemed not to collapse with time, then the well does not have a casing covering the earth formation where fluids are produced from. When used in horizontal wells, an uncased reservoir section may be installed in the last drilled part of the well. The well designs discussed here can be applied to vertical, horizontal and/or deviated well trajectories.
To produce hydrocarbons from an oil or natural gas well, a method of water-flooding may be utilized. In water-flooding, wells may be drilled in a pattern which alternates between injector and producer wells. Water is injected into the injector wells, whereby oil in the production zone is displaced into the adjacent producer wells.
The water pressure required in order to push the oil into the producer wells must overcome the fluid friction losses in the earth formation between injector and producer and must overcome the reservoir pressure minus the hydrostatic head of the injection fluid. The water pressure, possibly combined with a low water temperature e.g. in the order of 5 degrees C., can induce fractures in the rock of the reservoir formation. If a fracture extends from an injector well to a producer well, it may form a channel through which water may be conveyed directly from the injector well to the producer well therewith not pushing the oil or gas in front of the water to the oil or gas production well.
Water may also be conveyed through naturally occurring fractures in the earth formation and thereby not push the oil to the producing well.
Knowledge of the position of such water bearing fractures may in the prior art be determined by conveying a suite of petrophysical tools in the well to determine where water is located. This can be done in an open hole completion or after cementing a liner in the open hole.
However, cementing a liner in an open hole completion may be associated with a number of technical problems, such as for example: 1) the liner may run into an existing side track or a leg of a fishbone well; 2) cementation of the liner cannot be carried out due to losses; 3) the cementation causes fractures in the reservoir creating a connection to another well.
Conveying petrophysical tools into wells, especially horizontal wells is limited to the depth that can be reached with any means of conveyance suitable for particular well dimensions.
Thus, it may be advantageous to be able to identify such water bearing fractures without cementing a liner into the open hole completion and without having to convey petrophysical logging tools into horizontal wells by conventional means.
U.S. Pat. No. 6,241,028 disclose a method and system for measuring data in a fluid transportation conduit, such as a well for the production of oil and/or gas. The system employs one or more miniature sensing devices which comprise sensing equipment that is contained in a preferably spherical nut-shell. However, horizontal wells need not be straight. Further, wells may contain obstructions such as wash-outs and/or well side tracks, e.g. in fishbone wells, which may prevent the above system from examining the entire well.
Thus, it may be advantageous to be able to examine wells comprising obstructions such as wash-outs and/or side tracks and/or to be able to examine non-straight horizontal wells.
Therefore, an object of the invention is to enable examination of wells containing obstructions such as wash-outs and/or side tracks and/or non-linear well part in open hole completion parts of the well.